Weighing soil and water

The first job was to check the weight bearing capacity of the roof.

Can anyone guess what the weight of wet soil in this bed would be? There are calculation tables to help you work it out.

The first email that was sent when we decided to put raised beds on the cafe roof was the check the load bearing capacity of the balcony. We liaised with the engineers, GHW Consulting and the architects Stride Treglown who worked on the refurbishment of The Dolphin Centre to ask about our weight issues. The balcony is designed for a load of 5KN/sqm (500KG/sqm). We have 43sqm of space so we can carry 21,500kg. If the average adult weighs 100kg (that’s a heavy person) we can have 215 people weighing 100kg on the balcony (that’s impossible!) It did reassure us that we could easily put beds on the balcony.

To be extra safe we layered lightweight panels to reduce the amount of soil we needed. We wrestled over the materials, preferring a natural material for the layer. The reality however is that a natural material our eventually breakdown and we would be left with organic matter with the weight implications therein. So we used insulation panels that would withstand moisture.

So, what’s the weight of the raised bed I hear you think? Well the size is .80cm x 240cm x 40cm deep. The full wet soil calculation is 1,500kg. were only filling 50%-75% with soil making the average bed 1,000kg. We could have 21 beds on the roof but we only have 8 (8,000kg), leaving room for lots of heavy people to visit!

If you want to understand more about weight / soil calculations here is the table we used.

We used polystyrene and hard thermal insulation panels to line the beds so we didn't need as much soil and compost.